A wind power plant usually has a plurality of wind turbines for converting wind energy to electricity. To supply electricity to the electricity end users, the wind power plant is connected to a power grid. However, before the wind power plant can be connected to a power grid, the wind power plant has to meet the requirements of electrical performance of the wind power plant specified by grid codes. One requirement is an initial response time of the wind power plant for various parameters such as voltage, frequency, active power and reactive power.
A wind power plant is often referred to as a group of wind turbine generators which are commonly connected to an electrical grid through a common connection point, also known as Point of Common Coupling (PCC). Generally, the wind power plant has a wind power plant controller which monitors a power grid voltage and compares the power grid voltage with an external setpoint. A difference between the actual power grid voltage and the external setpoint (e.g. an error signal) is used to calculate a command for reactive power production for the wind power plant. This command is sent from the wind power plant controller to the individual wind turbines which in turn will respond (e.g. produce more or less power in order to adjust the power grid voltage) upon receiving the command. The error signal may be caused by a change in the actual power grid voltage or a change in the external setpoint. The initial response time of the wind power plant is to be understood as the time period starting upon detection of a change in the power grid voltage or a change in the external setpoint and ending as soon as the first wind turbine of the plurality of wind turbines of the wind power plant responds (as soon as response is detected).
Often the power plant controller operates with some restrictions. These restrictions can be referred back to current or voltage limitations, but in practice the limits are often implemented as a restriction to the reactive power supplied from the wind power plant to the grid through a point of common coupling. This restriction can either be a hard reactive power limit with a numerical value of VAr's or p.u. or a Power Factor value, meaning that the reactive power is limited to an upper maximum defined according to the present active power production.
In classical control theory with feedback loops it has been known to have a reference and compare it with a measurement. The difference between the reference and the measurement, also known as the error is used as the input for a controller. The controller is often a PID, PI or even P controller. Large errors often drives the controller to a less stable situation, as saturation may occur in the integral part of the controller or saturation occurs as maximum or minimum ceilings of the output values can be reached.